Such formed parts are employed for inner coverings in motor vehicles particularly in the region of the trunk. Duroplastics, especially phenolic resins, are used as binders. An illustrative example of this prior art is Eisel, D. "Recyclate aus Rei.beta.baumwolle und Phenoplast--ein bewahrtes System," Melliand Textilberichte, November 1992, pages 873 to 878.
Due to the low compression of the carrier fibrous web forming the core of the sandwich structure, such formed parts exhibit good acoustic properties However, these formed parts are not sufficiently rigid despite the sandwich structure, since the low compression carrier fibrous web is "weak to shearing," only restrictedly transmitting shearing forces as well, and thus not being ideal for the core of a sandwich construction. Hence, it has been common practice for quite some time to improve the rigidity of such formed parts, especially the package trays for which such increased carrying capacity is required. Such rigidity has been improved, by at least one of the following measures:
Incorporating reinforcing fibers such as glass fiber, flax fibers or high-strength synthetic fibers as well into the carrier fibrous web; PA0 Incorporating metal inserts into the carrier fibrous web; and PA0 "Hard" pressing of a greater marginal region, in order to gain more rigidity there by means of a greater marginal region and by a greater compression of material.
However, incorporating of reinforcing fibers means a greater manufacturing requirement. As far as natural fibers such as flax is concerned, it has to be reckoned with an odor emission; mineral fibers affect in case a considered recycling. The same applies for metal inserts too, which cause both a considerable expenditure in manufacturing and still a significant weight gain of the formed parts as well. The branch-used utilization of phenolic resins is uncertain with regard to the climate inside the seating compartment. Hence, the car industry more frequently demands for a substitute of duroplastics by thermoplastic binders, not least with regard to a better recycling capability of thermoplastic binders, as well.
A "hard" pressing of the marginal regions is only operative when these regions are adequately large. Usually, they comprise more than one third of the formed part surface. For improvement of the seating compartment acoustic, the "hard" pressed regions of formed part are no longer usable.